This invention relates to a vehicle height control system, and in particular to a highly reliable vehicle height control system in which a vehicle height control is inhibited for an apparent variation, i.e. a temporary change of the vehicle's height arising at the time when a vehicle is turning.
A vehicle height control system has been proposed in Japanese patent application laid-open No. 56-60711 in which the vehicle's height is adjusted to a target level by means of sensors capable of detecting the change of the vehicle's height. It is known that a vehicle's height is advantageously controlled as the load weight of a vehicle is changed. However, the vehicle's height also changes with the change of the surface of a road or during the turning of the vehicle during the running operation thereof, in spite of the fact that the load weight of the vehicle is not actually changed. To control the vehicle's height each time such an apparent change occurs does not improve performance as well as being uncomfortable to a vehicle's driver or passengers.
Therefore, it is required in a vehicle height control system that the vehicle's height be controlled as soon as the load weight is actually changed when the vehicle is stopped but that the vehicle's height not be controlled for apparent changes of the vehicle's height during the running of the vehicle.
To this end, one may consider that no vehicle height control should be carried out during the running mode thereof. However, the following problems will then arise:
(1) If the vehicle's height is controlled according to the change of the load weight when the vehicle is stopped and the vehicle is started before the vehicle height control has not yet been completed, the vehicle's height is changed owing to the acceleration thereof, resulting in the completion of the vehicle height control although the vehicle's height has not yet attained the target level according to the actual load weight. In this case, since the vehicle height control is not re-activated until the vehicle is again stopped, the vehicle height control is not yet completed;
(2) If an air leakage in the air suspension occurs during the vehicle's running for a long time interval, the vehicle's height is gradually reduced due to that air leakage and can not be corrected by the vehicle height control. Furthermore, multi-level or multi-stage control can not be carried out for different vehicle driving speeds, in which, for example, a lower vehicle's height is set for a high speed running.
As indicated above, if the vehicle height control is completely inhibited during the running mode of a vehicle, an effective vehicle height control is not achieved. Thus, there has been already proposed a system for solving the above problems in which the vehicle control is initiated immediately in response to the change of the vehicle's height in the stopped state while during the running the initiation of the vehicle height control is delayed by means of a timer or the time interval for determining the initiation of the vehicle height control is extended.
However, although this system can inhibit the initiation of the vehicle height control for the variation of road surfaces during the running mode, it can not inhibit said initiation in a case such as the turning of a vehicle where the vehicle's height is being changed for a relatively long interval. This is because if one intends to inhibit the vehicle height control from operating for changes in the vehicle's height during the time of long internal turning, the control system of the prior art must delay the initiation of the vehicle height control during the running mode by a long time interval or to extend the time interval for the determination of the vehicle height control whereby it will be difficult to initiate the actual vehicle height control when required. To the contrary, if one intends to surely carry out the normal or real vehicle height control during the running mode, it is advantageous in the prior art to shorten the delay time of the initiation of the vehicle height control or to shorten such a long time interval for the determination of the initiation of the vehicle height control. However in the prior art system, such a vehicle height control will be disadvantageously initiated even for the apparent change of the vehicle's height during the vehicle's turning for a long time interval.
According to such conventional systems, particularly during the running of a vehicle, it is difficult to distinguish between an actual change in the vehicle's height and a temporary change in the vehicle's height due to the turning of the vehicle and the like, so that if the vehicle height control is desired to be carried out quickly during the running mode of operation of the vehicle, it is difficult to inhibit the vehicle height control during the turning.
On the other hand, SAE paper 770396 on pages 11-22 discloses "Electronic Sensing for Vehicle Height Control" by R. W. Hegel et al, in which optical vehicle's height sensors and an air pressure source for the vehicle height control are employed for weight and space savings and simplified installation; SAE paper 840258 on pages 1-12 discloses "Chassis Electronic Control Systems" by M. Mizuguchi et al, in which optical vehicle's height sensors and an air pressure source are also employed while fail-safe mechanisms are incorporated in the event of system failure and a central diagnostic terminal is provided for system checking.